The present invention relates to an improved device for housing a planar optical component for use in chemical sensing for example.
New chemical sensor technologies using optical techniques (in particular interferometric techniques) are providing new high performance devices. Whilst these devices are relatively simple in terms of components, the tolerances required in the assembly-procedure can be extremely onerous. Of these, end illuminated interferometric devices are perhaps the most demanding. In such cases, sub-micron misalignment between the electromagnetic radiation source (typically a collimated, focussed laser) and the sensor substrate itself may be sufficient to prevent its correct operation.
There are several situations which may lead to distorted output from a conventional device. Thus, the light beam may pass over the top of the planar optical component and distort the output received by-the detector. Similarly, where the device comprises a planar sensing waveguide and a planar reference waveguide, if the light misses a waveguide or fails to illuminate both equally, the output may be lost or distorted. Thus, if any of the components (eg light source, lenses, polarisers, sensors etc) are misaligned by as little as 2xc3x9710xe2x88x927 metres (200 nm) the performance of the device will be adversely effected. The provision of a device which ensures that waveguides are illuminated equally without admitting stray light represents a significant technical challenge.
More generally, there is a need for sensor assemblies of simpler construction and improved reliability. The range and applicability of chemical sensors could be greatly enhanced if it were possible to achieve lower manufacturing costs and greater robustness. An important consideration in developing suitable devices is temperature management. This imposes various design constraints related to the thermal mass of areas requiring insulation and the disposal of unwanted heat into the environment.
The present invention seeks to provide an improved device for housing a planar optical component such as a chemical sensor which is-capable of ultra high precision temperature control. The device is advantageously robust and gives enhanced signal to noise ratios (sensitivity). Moreover, the invention seeks to provide an optical (interferometric) chemical sensor device which is simple to machine and assemble and fault tolerant in terms of construction errors and which may be used to obtain reliable information relating to the changes occurring within the device.
Thus viewed from one aspect the present invention provides a device comprising:
an optical assembly adapted to mount a planar optical component (eg a sensor) so as to define a longitudinal path through the device in which the planar optical component is effectively exposed in free space and including guiding means for correlating along said longitudinal path the position of said planar optical component and of a source of electromagnetic radiation, whereby to expose said planar optical component to said electromagnetic radiation along said longitudinal path whilst substantially eliminating stray electromagnetic radiation,
wherein the optical assembly comprises a cavity which permits access to a face of the planar optical component or a face of a base with which the planar optical component is in intimate thermal contact whereby to enable an inner temperature controller to be positioned in thermal contact with the planar optical component for controlling the temperature of the planar optical component.
The inner temperature controller is capable of permitting fine temperature control of the planar optical component (eg sensor) and may be a heat pump or thermo-electric controller capable of providing or removing heat as desired. In a preferred embodiment, the inner temperature controller is an inner Peltier assembly capable of adding heat to or dissipating heat from the planar optical component. The inner Peltier assembly may comprise an inner Peltier mounted on an inner Peltier mount. The inner Peltier mount conveniently provides thermal mass. Preferably, the Peltier mount has a concave underside to optimise thermal contact with the planar optical component (or its base). The inner Peltier and inner Peltier mount may be provided with suitable insulation as desired.
Preferably the planar optical component is a sensor. In a preferred embodiment, the sensor is mounted on a sensor base and is in intimate thermal contact therewith. The base is typically made of stainless steel which advantageously provides thermal mass. Preferably the optical assembly is thermally insulating to permit the sensor, sensor base and Peltier mount to be in intimate thermal contact with the inner Peltier and thermally isolated from other components of the device.
Preferably the device is provided with a Peltier exhaust assembly which permits thermal transfer from the exhaust side of the inner Peltier to the environment.
Preferably the Peltier exhaust assembly comprises an exhaust plate positioned to allow thermal exchange with the environment. The exhaust plate is conveniently located at or near to an end of the device remote from the optical assembly. Preferably the Peltier exhaust assembly comprises means for thermally contacting the inner Peltier assembly with the exhaust plate. A thermally conducting strip may be used for this purpose (eg of copper). Preferably, the Peltier exhaust assembly comprises an exhaust guide (eg in the form of a ring) which is capable of fitting over the insulating collar of the laser module. The exhaust guide defines a slot into which the exhaust strip may be inserted.
In a preferred embodiment, the optical assembly and inner temperature controller are contained within a conducting sleeve. The conducting sleeve fulfils thermal management of the temperature sensitive components of the device eg provides a highly stable temperature environment for the inner temperature controller, provides precision temperature control for peripheral components such as the laser diode, provides a thermally stable environment for temperature control electronics and controls the temperature of incoming gases or liquids through the inlet and outlet ports. All these functions contribute to the temperature of the planar optical component being contained within desirable limits (typically the target control span is 20 micro Kelvin).
In a preferred embodiment, the conducting sleeve comprises a heat shroud which is typically made of copper. The heat shroud is preferably provided with an opening which is suitably disposed to coincide with the cavity in the optical assembly. This advantageously allows the inner Peltier assembly to be inserted in the optical assembly, after the optical assembly has been inserted in the conducting sleeve (eg heat shroud).
Preferably, the heat shroud comprises an integral laser module holder for inserting a laser module. Preferably the laser module holder is provided with an outwardly disposed insulating collar. Preferably the electronics are housed within the heat shroud.
Preferably the device comprises an outer temperature controller-which permits-coarse temperature control of for example the conducting sleeve, laser module, laser module holder, the exterior parts of the optical assembly and the electronics. The outer temperature controller is thermally independent of the inner temperature controller. The outer temperature controller conveniently takes the form of an outer Peltier assembly. Preferably, the outer Peltier assembly is provided externally of the restraining sleeve which is provided with an aperture to enable exposure of an effective area of the conducting sleeve to achieve thermal contact with the outer Peltier assembly.
Preferably, the device is provided with a means for urging the Peltier exhaust assembly onto the inner Peltier assembly. For example, a restraining sleeve is added outwardly of the heat shroud to force the Peltier exhaust assembly onto the inner Peltier assembly at one end and the exhaust plate at the other.
A preferred device of the invention is based on the principle of xe2x80x9ca Russian dollxe2x80x9d which has the advantage of being able to be constructed from a plurality of discrete assemblies. The assemblies of the device may be constructed as a plurality of shells which allow advantageously straightforward, sequential construction of the overall device.
Thus a preferred embodiment of the device of the invention is capable of sequential construction from a plurality of discrete assemblies, said assemblies being: an optical assembly contained within a conducting sleeve; an inner Peltier assembly comprising an inner Peltier; and a Peltier exhaust assembly,
wherein: the inner Peltier assembly is housed within the cavity of the optical assembly so-as to achieve intimate thermal contact with the planar optical component and the Peltier exhaust assembly permits thermal transfer from the exhaust side of the inner Peltier to the environment and is thermally isolated from the inner Peltier assembly and conducting sleeve.
Particularly preferably, this embodiment further comprises a discrete assembly being an outer Peltier assembly in thermal contact with the conducting sleeve.
Viewed from a further aspect the present invention provides a kit capable of being assembled into a device as hereinbefore defined, said kit comprising:
an optical assembly, an inner Peltier assembly, a conducting sleeve, a Peltier exhaust assembly and an outer Peltier assembly, wherein:
the optical assembly is capable of being inserted in the conducting sleeve;
the inner Peltier assembly is capable of being housed within the cavity of the optical assembly so as to achieve intimate thermal contact with the planar optical component; the Peltier exhaust assembly is capable of being-positioned in thermal isolation from the conducting sleeve so as to permit thermal transfer from the exhaust side of the inner Peltier to the environment; and the outer Peltier assembly is capable of being positioned so as to achieve thermal contact with the conducting sleeve.
Viewed from a yet further aspect the present invention provides the use of a device or kit as hereinbefore described as a gas or liquid sensor.
Viewed from a still yet further aspect the present invention provides a process for constructing a device as hereinbefore defined comprising the steps of:
inserting an optical assembly in a conducting sleeve (eg copper heat shroud) comprising an integral laser module housing;
inserting a laser module into the laser module housing;
housing an inner Peltier assembly in the cavity of the optical assembly so as to achieve thermal contact with the planar optical component;
positioning a Peltier exhaust assembly in thermal isolation from the conducting sleeve so as to permit thermal transfer from the exhaust side of the inner Peltier to the environment.
Where appropriate, the process of the invention may comprise the additional steps of:
constructing an outer restraining sleeve;
constructing an outer casing; and
positioning an outer Peltier assembly on the outer casing or restraining assembly whereby to achieve thermal contact with the conducting sleeve.
In order to impart optimum thermal performance to the device, the materials of the various component parts are judiciously chosen. Where necessary, component parts may be required to have good insulating and mechanical properties, thermal drive (good thermal conductor), thermal exhaust (good thermal conductor), high performance insulating properties and mechanical properties, high performance insulating properties, etc. Materials for these purposes will be, familiar to those skilled in the art.
The exclusion of stray radiation in accordance with the optical assembly of the device of the invention enables the number of components to be minimised and enables straightforward analysis of the signals generated by the planar optical component (such as the centre of gravity of a series of interferometric fringes for example). This is achieved by ensuring that electromagnetic radiation excites substantially only the planar optical component. The optical assembly of the device of the invention is suitable for the fault tolerant construction of planar optical sensors and ensures optimal performance from the planar optical component. Tolerances are typically reduced by approximately 1000 fold enabling cheap mass production methods such as compression moulding and injection moulding to be employed.
In a preferred embodiment, the optical assembly of the device according to the invention comprises a planar optical component having a plurality of waveguides. Typically the planar optical component comprises a sensing waveguide and a reference waveguide. Preferably the planar optical component is any of those described in WO-A-98/22807 (IMCO (1097) Ltd et al).
Preferably the optical assembly is provided with one or more seats upon which the planar optical component may be seated.
The optical assembly may comprise a holder for mounting the planar optical component and a housing adapted to receive internally said holder so as to define a longitudinal path through the device in which the planar optical component is effectively exposed in free space. Preferably the holder comprises a basal recess in which the planar optical component may be mounted. To ensure that the edge of the planar optical component which is to be excited by the electromagnetic radiation is suitably exposed in the longitudinal path, one or more longitudinal cavities may be provided in the base of the holder such that when the planar optical component is positioned adjacent an aperture in the housing, the majority of the leading and trailing edges of the planar optical component may be exposed in free space. In a particularly preferred embodiment of the invention, the holder is removably received in the housing. The provision of a holder of this type advantageously enables the planar optical component (eg sensor) to be replaced without discarding or rebuilding the supporting components. Where appropriate, the optical assembly of the device of the invention may provide a means for providing a constant force between the holder and the housing.
Preferably the optical assembly of the device of the invention includes a guiding means in the form of a spacer incorporated in the planar optical component or in the main body of the optical assembly. In the first instance, the spacer may be incorporated in the planar optical component conventionally during manufacture. In the second instance, the spacer takes the form of (or is located on) a seat in the optical assembly upon which the planar optical component is located in use. This latter embodiment has the advantages that the sensing layer of the planar optical component is more efficiently exposed to the test material, that the manufacture of the planar optical component is simplified and that the disturbance of the planar optical component (as a result of bringing it into contact with the seat or with the modified seat upon which the spacer is located) is minimised. The material from which the spacer is made is judiciously chosen in terms of refractive index and physical properties. The spacer is advantageously permeable to the sample under analysis.
In a preferred embodiment, the optical assembly of the device of the invention comprises a first aperture at a first end of a longitudinal path for admitting electromagnetic radiation and a second aperture at a second end of said longitudinal path for transmitting electromagnetic radiation. Provided the spacer is of a known predetermined thickness relative to the known distance between the first aperture and the surface upon which the planar optical component is seated within the optical assembly, electromagnetic radiation may be effectively guided onto the waveguides.
In a particularly preferred embodiment, the planar optical component and incorporated spacer may be located on a silicon baseplate. The silicon baseplate which is typically optically flat may be conveniently provided with a hole over which the planar optical component is located. Conveniently, the spacer may seal the hole in the baseplate provided the spacer is sufficiently (eg optically) flat.
In an especially preferred embodiment, the silicon baseplate is provided with a channel (eg a V-shaped channel) capable of receiving an optical fibre wherein the depth of the channel predetermines the position and height that electromagnetic radiation is emitted relative to the surface of the silicon baseplate. Since the position of the waveguides above the surface of the silicon baseplate is determined by the height of the incorporated spacer, the position of the electromagnetic radiation and the waveguides may be correlated. Stray light is simply emitted into the silicon.
In an alternative especially preferred embodiment, the silicon baseplate forms part of an integrated electro-optic device in which a laser source is integrated into the silicon baseplate. The guiding means is provided by an incorporated spacer located on the silicon baseplate or the planar optical component as hereinbefore described.
In either of the especially preferred embodiments, the output may be monitored by a discrete detector or an imaging fibre or fibre array may be used to collect the output image. Alternatively, a photodetector could be integrated into the silicon structure. Using fibres in and out is very useful in safety critical applications (ie there is no electricity).
Preferably, the optical assembly of the device of the invention comprises means (eg a flat surface, one or more seats or seals) for providing a gas or liquid seal to the surface of the planar optical component to allow transport of an analyte to and from the planar optical component (eg sensor) and measurement of the optical behaviour of the component in the presence of the analyte. The provision of a seal to the surface of the planar optical component (eg sensor) reduces the dead volume to a minimum (this is important in providing optimal performance with chemical sensors). The provision of a seal to the surface of the planar optical component (eg sensor) also enables liquid samples to be used in addition to gas samples. This is not conceivable with a conventional freestanding arrangement as wetting of the end faces would lead to optical misalignment.
Preferably, the optical assembly of the device of the invention is capable of mounting an electromagnetic radiation source such as a laser. Preferably, the optical assembly is capable of mounting an electromagnetic radiation detection device (eg photodiode array). Preferably, the optical assembly of the device of the invention comprises means for the provision of removable or non-removable components between the planar optical component (eg chemical sensor) and a source of electromagnetic radiation and/or between the planar optical component and a radiation detection device. Such components may be conventional lenses, polarisers, electromagnetic radiation windows, spacers, window/spacer retainers, etc mounted in a conventional manner.
In all cases, the body of the optical assembly is preferably opaque to minimise stray electromagnetic radiation. Thus the planar optical component may be advantageously mounted on a base which does not transmit electromagnetic radiation, thereby preventing stray electromagnetic radiation passing thereunder. Preferably, the seat or seal of the optical assembly also may not transmit electromagnetic radiation in the longitudinal direction whereby to further prevent stray electromagnetic radiation passing over the sensor surface and reaching the detector. Preferably, the seat or seal has an inlet, a channel and an outlet providing a means through which analyte (eg gases or liquids) may pass. In this way, analyte is able to pass into and out of the absorbent layers of a planar optical component (eg chemical sensor) leading to measurable changes in the output electromagnetic radiation. Preferably, the extremes (edges) of the sensor are sealed from the environment to prevent extraneous effects from gases, vapours or liquids from external sources not related to the sample under analysis.
In a preferred embodiment, where the device of the invention is to be used on test materials, it is preferred that the thermal mass of the incoming material is minimised by ensuring the sample volume and the inlet volume are minimised. This may be achieved by low dead volume within the sensor xe2x80x9ccavityxe2x80x9d and narrow bore inlets. In addition, the thermal mass of the inlet system needs to be high to prevent thermal fluctuation over time. Stainless steel pipework is preferred. Preferably, the inlet pipe is in thermal contact with the copper shroud eg the pipework is run along the shroud. Appropriate thermal lagging of the pipework may be required in order to prevent too high a thermal loss from the complete outer system.